It is often times necessary, or at least desirable, that the location of objects, including personnel, be known or at least be determinable. This is particularly true, for example, in the case of underground miners.
While various attempts have been made to monitor the position of objects, including personnel, and/or to provide an indication of location, such attempts have not been heretofore completely satisfactory due at least in part to the problems associated with underground mines, the necessity of not unduly burdening the miner and/or providing a relatively simple yet dependable system.
With respect to the prior art patents, various systems have been heretofore suggested for identification of personnel or other objects (see, for example, U.S. Pat. Nos. 4,209,783; 4,223,830; and 4,242,663) and for surveilance of particular areas (see, for example, U.S. Pat. Nos. 3,257,653; 4,095,211; and 4,229,737). In addition, it has heretofore been suggested that a coded signal could be transmitted to a plurality of remote transceivers for activating transponders when a unique code was received by the transponder (see U.S. Pat. No. 3,805,265). Also, it has heretofore been suggested that a pair of pulses can be transmitted to activate a transponder depending upon a preselected spacing between pulses (see U.S. Pat. No. 3,226,680).